Open your published page, and click Publishing Tools in the top navigation bar

Click the Development Articles link in the left navigation to open your in-development Instant Articles

Create a new article, or edit an existing one

The article editor will open

The markup in the above image is taken from Facebook's sample Instant Article markup. An 'op-tracker' iframe must be added to the markup as in the above image:

StatCounter is now active on your Facebook Instant Article and will immediately begin recording visitor information. You can preview your article in the Facebook Pages Manager app and verify traffic is being recorded,